The growing information transmission rate is one of major problems which radio communication systems in the future will face. In order to realize this goal with limited spectrum resources, a multi-antenna technology (for example, MIMO: Multi-Input Multi-Output) is becoming one of indispensable means used in radio communication systems for now and future. In a MIMO system (or referred to as a “system”), the transmitting side transmits a signal using multiple antennas and the receiving side receives a signal in space using multiple antennas. According to researches, compared to a conventional single antenna transmission scheme, the MIMO technology can improve channel capacity significantly and improve information transmission rate.
FIG. 1 schematically shows the configuration of a conventional MIMO system.
In this configuration, the transmitting side and the receiving side transmit and receive a signal using nT transmit antennas 105 and nR receive antennas 106, respectively.
Data subjected to transmission on the transmitting side is outputted from data source 101 to S/P (serial/parallel conversion) section 102 first and divided into nT data substreams by S/P section 102. The divided nT data substreams correspond to nT transmit antennas 105 respectively.
Each data substream is subjected to CRC (Cyclic Redundancy Check) encoding by CRC encoding sections 103 before transmission. Here, CRC encoding refers to adding a CRC bit to the rear of each block of the data subjected to transmission. The CRC bit is used to verify a corresponding block on the receiving side. That is, when CRC decoding sections 108 on the receiving side performs CRC decoding, it is possible to determine whether or not errors occur in blocks.
Each data substreams after CRC encoding is subjected to channel encoding and modulation by channel encoding and modulating sections 104. Each data substreams after encoding and modulation is subjected to predetermined radio transmitting processing including, for example, digital-to-analog conversion and up-conversion and transmitted from corresponding transmit antennas 105.
On the receiving side, all signals in a space are received by nR receive antennas 106 first and the received signal is subjected to predetermined radio receiving processing including, for example, down-conversion and analog-to-digital conversion. When a pilot signal is added to data on the transmitting side, the pilot signal is extracted from the received signal. Furthermore, channel estimating section 110 performs channel estimation based on the pilot signal or using other schemes so that it is possible to obtain present channel characteristic matrix H (for a MIMO system, the channel characteristic can be represented by one matrix).
MIMO detecting/retransmission combining section 107 subjects the signal after radio receiving processing to MIMO detecting processing and combining processing. MIMO detecting processing uses channel characteristic matrix H and subjects signals received by receive antennas 106 to detecting processing. Furthermore, combining is performed on the signal version after the same signal retransmission. The signal finally obtained is outputted to CRC decoding sections 108.
Here, various methods can be used in detecting MIMO. For example, ZF (Zero Forcing), MMSE (Minimum Mean Square Error), SIC (Successive Interference Cancellation) or other methods are often used.
MIMO detection generally includes two operations. Namely, the operation of decomposing a received signal using a detection element and obtaining signals corresponding to data substreams transmitted from transmit antennas 105, and the operation of demodulating and decoding the signals. These two operations are not performed independently. That is, the output from the former is acquired by the latter, and the progress of the former often requires the output from the latter. Therefore, demodulation and decoding will be described included in MIMO detecting/retransmission combining section 107 for convenience.
The signal outputted to CRC decoding sections 108 corresponds to the data substreams after CRC encoding and before channel encoding and modulation.
CRC decoding sections 108 subject the signals outputted from MIMO detecting/retransmission combining section 107 to CRC decoding. Thus, it is possible to determine whether or not each present data substream is received correctly.
Furthermore, CRC decoding sections 108 generate a positive acknowledgement signal (ACK) or a negative acknowledgement signal (NACK) after every CRC decoding.
The generated ACK/NACK signal is fed back to the transmitting side through feedback channel 111. Here, an ACK signal refers to a signal where no code error occurs in a data block and a NACK signal refers to a signal where some code error occurs in a data block. When the transmitting side determines that a signal fed back from the receiving side to the transmitting side is an ACK signal, this means that the corresponding data substream is correctly received, and the transmitting side transmits a new data substream from corresponding transmit antennas 105 upon next transmission. Further, when the transmitting side determines that the signal fed back is a NACK signal, this means that the corresponding data substream after receiving processing contains some code error, and the transmitting side retransmits the original data substream from corresponding transmit antennas 105 upon next transmission.
When the CRC decoding result confirms that the data substream is received correctly without error, the CRC bit added to the rear of data by CRC encoding sections 103 on the transmission side is removed and the original data stream is obtained.
Data streams outputted from CRC decoding sections 108 are subjected to parallel-to-serial conversion by P/S conversion (parallel-to-serial) section 109, and finally received data is acquired and outputted.
As described above, MIMO detecting/retransmission combining section 107 carries out MIMO detecting and combining processing on received data after a plurality of transmissions of the same data stream. For example, after a certain data stream is transmitted N times (one of them is the first time transmission and N−1 are the number of times of retransmissions), N received signals are acquired. The received signals are represented as r1, r2, . . . rn. r1 among them is a received signal after the first transmission and the rest are received signals after retransmissions. These received signals also normally different each other according to channel characteristics changes and differences in noise per transmission. The receiving side subjects these N received signals r1, r2 . . . rn to combining processing and when correct substreams still are not acquired at this time, next retransmission is requested. Furthermore, after next retransmission signal is received on the receiving side, (N+1) received signals r1, r2 . . . rn, rn+1 are subjected to combining processing. In the MIMO system, combining processing of a plurality of received signals generally includes two methods. (1) Performing detection and then combining. That is, received signals are individually subjected to MIMO detection and the detected signals are then combined. (2) Performing combining and then detection. That is, a received signal is subjected to combining first and the combined signal is then subjected to MIMO detection.
In a conventional MIMO system, when an error is found in a data substream transmitted from a certain transmit antenna and the substream data is retransmitted, generally no antenna selection is performed. When an error occurs in a substream of one of antennas the conventional MIMO system actually uses a method for retransmitting the substream through the antenna. In this way, antenna selection is not performed in every retransmission in the conventional MIMO system, and the following situation may occur. (1) Antenna characteristics upon retransmission are so poor that, even when the receiving side performs combining processing after this retransmission, a completely correct data stream cannot be acquired. Therefore, next retransmission needs to be performed, which affects the time delay characteristic of the system. (2) Antenna characteristics are so excellent upon retransmission that retransmission could actually may be realized even when a worse antenna is selected. However, no selection is made here and only a poor antenna is used for transmitting new data, it naturally causes degradation of the code error characteristic in the system.